1. Field of the Invention
The present invention relates generally to the field of downhole sampling and in particular to the downhole analysis of fluids and gases containing hydrocarbons using electromagnetic radiation (light) including Mid Infrared (MIR) light, Near Infrared (NIR) light and a temperature sensitive detector, such as a pyroelectric detector for measurement and estimation of properties of fluid and gas samples and properties of the reservoir from which a fluid or gas is taken, prior to, during, or after capture of the fluid or gas in a sample chamber.
2. Summary of the Related Art
In wellbore exploration, drilling mud such as oil-based mud and synthetic-based mud types are used. The filtrates from these mud types generally invade the formation through the borehole wall to an extent, meaning that these filtrates must be removed, as much as they can be removed, from the formation by pumping in order to access the formation fluids after filtrate has been pumped out. Open hole sampling is an effective way to acquire representative reservoir fluids. Sample acquisition allows determination of critical information for assessing the economic value of reserves. In addition, optimal production strategies can be designed to handle these complex fluids. In open hole sampling, initially, the flow from the formation contains considerable filtrate, but as this filtrate is drained from the formation, the flow increasingly becomes richer in formation fluid. That is, the sample flow from the formation contains a higher percentage of formation fluid as pumping continues.
It is well known that fluid being pumped from a wellbore undergoes a clean-up process in which the purity of the sample increases over time as filtrate is gradually removed from the formation and less filtrate appears in the sample. When extracting fluids from a formation, it is desirable to quantify the cleanup progress, that is, the degree of contamination from filtrate in real time. If it is known that if there is too much filtrate contamination in the sample (for example, more than about 5 or 10%), then there may be no reason to collect the formation fluid sample into a sample tank until the contamination level drops to an acceptable level. Thus, there is a need for a method and apparatus for directly analyzing a fluid sample and determining percentage of filtrate contamination in a sample.
Properties of formation fluids and gases have been determined in situ downhole using near-infrared light detection and analysis. Mid-infrared (MIR) light detection and analysis, however, has not been performed downhole even though the mid-infrared or “fingerprint” region of the spectrum is often preferable for identifying specific chemical compounds and for achieving higher sensitivity to small concentrations of chemicals. It has not been performed downhole primarily because of the difficulty of performing MIR spectroscopy in the downhole environment. The tool itself is very hot so it is continually emitting background MIR radiation, which could interfere with any readings taken by typical photodetectors. However, pyroelectric detectors respond only to changes in light intensity so they ignore any constant background of light radiation regardless of how intense such constant light is. Instead, they will respond only to a flickering light source. Another challenging part of light detection in a downhole tool (such as a downhole spectrometer) is the effect of the high downhole temperatures (up to 200 C) on typical photodetectors. For the same amount of light, the response of most photodiodes drops rapidly with increasing temperature because the internal shunt resistance of the photodiode drops as the temperature increases. The effect is exacerbated for longer wavelength photodiodes such as those sensitive to light in the 1.1 to 2.2 micron range and beyond, for example, in the MIR (2.5 to 11 micron) range. Thus, there is a need for a MIR detector suitable for use downhole.
Typically, the longer the wavelength that a photodiode can detect, the lower the photodiode's shunt resistance at room temperature. This shunt resistance drops even further at elevated temperatures. Thus there is a need for an optical detector that does not exhibit this shunt resistance problem at high downhole temperatures. Pyroelectric detectors respond to the rate of temperature change (such as that caused by absorbing a blinking light) rather than to temperature itself. Thus, pyroelectric detectors are not affected by high temperatures whenever those high temperatures are far below the detector's Curie temperature (which is 620 C for Lithium Tantalate). Light detectors are classified either as quantum detectors (photoconductors and photodiodes) or as thermal detectors (pyroelectrics, Golay cells, bolometers, thermopiles, some liquid crystals, etc.).
Quantum detectors are semiconductor devices that have a bandgap. Their conductivity changes when they absorb a photon that has enough energy to promote an electron from the valence band across the bandgap to the conduction band. The longest wavelength of light that a quantum detector can detect corresponds to light whose quantum energy is exactly equal to the bandgap energy. Mid-infrared light is low energy light so it can only be detected by small-bandgap quantum detectors. Unfortunately, the smaller the bandgap, the more likely it is that, at elevated temperatures, some electrons will have enough thermal energy to reach the conduction band even when no light is being absorbed by the detector.
Photoconductors are typically heavily N and P doped semiconductors such as lead sulfide or lead selenide. Exposure to light creates additional conduction electrons and holes, which cause the detector's resistance to drop. A small increase in the detector's ambient temperature usually creates a comparable increase in electron-hole pairs so these detectors are usually used with modulated light.
Both PN and PIN junctions are light sensitive. Such junctions are used to make photodiodes. When used in the photovoltaic mode, a photodiode generates current when it absorbs light. When used in the photoconductive mode, a reverse bias voltage is applied to the photodiode so that, when it is absorbs light, diode resistance drops and current flows in the reverse direction through the diode.
Thermal detectors detect light from the temperature changes they undergo when they absorb or release heat. Several types of thermal detectors are described below. A pyroelectric detector's response is proportional to its rate of temperature change when it absorbs modulated light. The reason is that, when a pyroelectric material is heated by a light pulse, its dipole moment changes, and while its dipole moment is changing, there is a temporary flow of current. A steady light produces no pyroelectric detector response regardless of the light's intensity.
A Golay cell is a thermal detector based on photoacoustics. Conceptually, it is a sealed, gas-filled box that absorbs light. Modulating the light causes pressure pulses in the gas within the Golay cell and these pressure pulses are picked up by a microphone. A steady light produces no Golay cell response regardless of the light's intensity.
A bolometer is a device whose electrical resistance changes due to heating caused by absorbing light. The two types of bolometers are the barretter (for which electrical resistance increases with increasing temperature) and the thermistor (for which electrical resistance decreases with increasing temperature). The term “thermistor” is often used to refer to both barretters and traditional thermistors. The qualifiers, “positive thermal coefficient” and “negative thermal coefficient”, respectively, are used to distinguish between the opposite directions of resistance change with increasing temperature.
A thermopile is a group of thermocouples connected in series. Each thermocouple is a junction of dissimilar metals that produces a voltage when one side of the junction is at a different temperature than the other side. A liquid crystal thermal detector makes use of the temperature-dependence of a liquid crystal's light scattering properties. The detector can be a thin plastic strip, covered with liquid crystals such as the disposable medical thermometers that are placed on a person's forehead.
Thermal detectors such as bolometers, thermopiles, and liquid crystals, generate a large steady-state signal due to the ambient temperature (or due to an above-ambient temperature caused by absorbing steady-state light) and a small modulated signal from transient heating caused by absorbing modulated light. Thus, they respond to background infrared radiation, steady-state infrared light, and to modulated infrared light. In principle, such thermal detectors could also be used to detect modulated infrared light in the hot downhole environment by processing their signal to remove the steady-state component and recover only the modulated component.
Thermal-change detectors, such as pyroelectric detectors and Golay cells, are probably more suitable than other thermal detectors for use as infrared light detectors in the hot downhole environment because thermal-change detectors generate no signal from the high ambient temperature or from steady-state light but respond only to modulated light.